The present invention relates to the technique of prestressed concrete and more particularly concerns reinforcements and devices for effecting a pretension for the mass-production of prestressed concrete elements. It also relates to such elements and in particular railway sleepers or ties.
Many reinforcing and pretension devices are known of the type comprising: an outer tube constituting the reinforcement proper closed at one end and open at the other; an inner core received in the tube; and means for putting under tension disposed at the open end of the tube and co-operating with the open end and the adjacent end of the core for establishing and maintaining between the tube and core an axial force of given value. Such arrangements are described in French Pat. No. 1,288,878, the first Addition No. 78,223 to French Pat. No. 1,263,984 and the German Pat. No. 522,510.
The first of these references provides means for putting the tube under tension which either weaken the tube in that they require it to be screwthreaded at both ends or are relatively elaborate and do not lend themselves to a profitable production on an industrial scale.
The two other references relate to two structures which employ for the core a material such as sand, mortar or concrete whose use does not lend itself well to mass-production and which does not permit obtaining characteristics that are idential from one element to another.